This invention relates in general to magnetic cores and core coil assemblies for electrical inductive apparatus, such as distribution transformers, and more specifically to a new and improved amorphous metal magnetic core construction.
Amorphous metal alloys, such as Allied Metglas Products 2605SC and 2605S-2, exhibit a relatively low no load loss when used in the magnetic core of an electrical transformer. Thus the user of amorphous metal alloys appears to be an attractive alternative to conventional grain oriented electrical steel in the construction of magnetic cores for electrical distribution transformers. Although amorphous metal has a higher initial cost then conventional grain oriented electrical steel, the cost difference may be more than offset over the operating life of a transformer by the savings in energy which otherwise would have to be generated to supply the higher losses.
Amorphous metal alloy, however, cannot simply be substituted for conventional electrical steel in the transformer manufacturing process. Amorphous metals possess characteristics which create manufacturing problems which must be economically solved before production line transformers utilizing amorphous metal cores will be readily available in the market place.
For example, amorphous metal is very thin, having a nominal thickness of about 1 mil. Amorphous metal is also very brittle, especially after stress relief anneal, which anneal is necessary after the core is formed of amorphous metal because amorphous metals are very stress sensitive. The no load losses of amorphous metals increase significantly after being wound or otherwise formed into the shape of a magnetic core suitable for distribution transformers. The no load loss characteristic is then restored by the stress relief anneal.
The thin, brittle amorphous metal strip also makes the forming of the conventional core joint a difficult manufacturing problem. While the use of a jointless core solves the joint problem, it complicates the electrical windings. Conventional electrical windings, which are simply slipped over the core legs before the conventional core joint is closed, cannot be used with an unjointed core. Techniques are available for winding the high and low voltage windings directly on the legs of an uncut amorphous core, but, in general, these techniques add manufacturing cost and production line complexity.
Conventionally, a core is formed by winding the core material on a mandrel in the form of a spiral. If a jointed core is contemplated, it is conventional to cut the core along a datum line which is to say that the core is cut straight through along a single radius. If the core is then opened and the high voltage and low voltage coils slipped over the legs and the joint rejoined a butt joint is accomplished with its attendant impediments to the flow of magnetic flux. One solution to this problem is disclosed in Ellis U.S. Pat. No. 3,107,415 in which, after the datum line cut the laminations are moved relative to each other to form a step lap joint from a series of concentric cylinders thus providing a flux path around the butt joints. Another alternative construction involves the datum line cutting of the core with the circumference of the core then slightly reduced so that each lamination or each group of laminations overlap the adjacent lamination or group of laminations to form a lap joint. The disadvantage of this construction is a substantial material buildup in the joint area of the core as well as undesirable air gaps being left adjacent the ends of each lamination or group of laminations.
As will be apparent from the foregoing a core joint is desirable which will avoid the necessity of expensive winding equipment required where a jointless core is used but which will provide as nearly as possible the electrical advantages of the jointless core without having to handle each lamination of the very thin amorphous metal individually, prevent the creation of air gaps in the joint area of the core as well as significant core height buildup in the joint area.